Investigate the significance of the fact that effector cells from healthy individuals and cancer patients are normally cytotoxic to cultured human tumor cells: by examining the level and specificity of the reaction directly and by competitive inhibition with effector cells isolated from healthy individuals and patients; by studying the level of cells bearing Fc receptors in the circulation and how these levels are regulated and affected by cancer; and documenting preliminary observations that natural antibodies can arm N cells; and by studying the levels and specificity of these natural antibodies.